420 PLANT MORPHOLOGY 



developed as a response to air exposure. Although the bryophytes are 

 thought to have arisen from chlorophycean ancestors, existing green algae 

 have unicellular sex organs. Therefore it is necessary to assume that the 

 ancestral forms had multicellular gametangia of the Ectocarpus type. 

 The bryophyte antheridium could readily have been derived from such a 

 gametangium by sterilization of the outer layer of cells. The archego- 

 nium, having diverged more widely from its original condition, went 

 through several stages in its evolution. At first it may have resembled 

 the antheridium, consisting of a group of fertile cells enclosed by a sterile 

 jacket. Further progress may have been marked by reduction of the fer- 

 tile cells to a single row and then by sterilization of all of these except the 

 lowest one, the other cells in the row becoming canal cells. Evidence for 

 this theory comes from the occasional appearance, in both liverworts and 

 mosses, of reversionary archegonia with multiple eggs, with two rows of 

 canal cells, or with some of the canal cells replaced by spermatogenous 

 cells. 



The sex organs of bryophytes and pteridophytes perform accessory 

 functions related to gametic union and embryo development. The sterile 

 jacket of the antheridium not only protects the developing sperms but 

 facilitates their dispersal. Frequently dehiscence occurs suddenly and 

 the sperms are discharged with considerable force. The neck of the 

 archegonium serves as a passageway for the entrance of sperms, the canal 

 cells breaking down to form mucilage through which the sperms swim. 

 The venter of the archegonium enlarges after fertilization, protecting the 

 embryo and aiding in the transfer of food to it. 



Antheridia with a large number of spermatogenous cells and archego- 

 nia with many neck canal cells are generally regarded as primitive. 

 Throughout the bryophytes and pteridophytes the tendency to reduce the 

 number of these cells reaches an extreme in the heterosporous pterido- 

 phytes, where the antheridium may produce only four sperms, as in 

 Isoetes, and the archegonium has only one neck canal cell. These trends 

 are continued into the spermatophytes, w^here antheridia are not organ- 

 ized and only two sperms or their equivalent are formed and where arche- 

 gonia, without any neck canal cells, are present only in the gymnosperms. 



The embryo sac of angiosperms may have evolved from the typical 

 female gametophyte of gymnosperms, but, except for the formation of free 

 eggs in the Gnetales, intermediate stages are lacking. Although most 

 angiosperms possess an eight-nucleate embryo sac that develops in a 

 characteristic way, many deviations from the typical pattern occur. 

 These reveal several trends, such as the participation of more than a single 

 megaspore nucleus in the formation of the embryo sac and a reduction in 

 the number of nuclear divisions that intervene between the formation of 

 the megaspore nuclei and the egg nucleus. 



